Heretofore, as absorbable surgical sutures, cat guts reproduced from the intestines of sheep have long been known and, recently, polyglycolic acid sutures have been developed. Although these materials are now in wide-spread use for surgical operations, they fail to meet all the requirements of absorbable surgical sutures. For example, cat guts have disadvantages in that they are inferior in ease of use, i.e., handling such as sewing and knotting is poor. Furthermore, their adaptability in vivo is poor because an antigen-antibody reaction may easily occur. In addition, the strength of the sutures is reduced because the sutures must be promptly absorbed in vivo. Also, polyglycolic acid sutures have disadvantages in that handling such as sewing and knotting is poor because of their high coefficient of surface friction. Furthermore, it is difficult to store them for a long period of time since they are readily decomposed in air, and in that the resistance to bacteria is poor, resulting in ready decomposition.
Chitin is a polysaccharide comprising poly(N-acetyl-D-glycosamine) and occurs widely in nature. For example, chitin may be found in the hard shell of crustaceans. Since chitin contains one aminoacetyl group per the recurring unit thereof, it has many interesting and peculiar characteristics. For example, it is absorbed in the tissue after undergoing enzymatic decomposition in vivo, and it has good dyeability. It has therefore been proposed to form chitin into fibers and use them as absorbable surgical sutures. However, absorbable surgical sutures having sufficient performance for practical use have not yet been produced from chitin fibers.
Absorbable surgical sutures must have good adaptability in vivo and be capable of being absorbed in the tissue after holding their tensile strength for a predetermined period of time and being sterilized. In addition, it is required that they have good handling such as ease of sewing and ease of knotting, and have suitable physical properties, such as tensile strength and flexibility, as sutures.
Sutures are usually prepared by twisting a plurality of filaments into threads or strands. In order to enable sutures to have the performance as described above, particularly good handling, i.e., ease of use, and suitable physical properties as sutures, it is necessary that fibers constituting the suture have suitable tensile strength and thickness. In more detail, these fibers are required to have a dry tensile strength of at least 2 g/d and a thickness of 20 denier or less, preferably 0.5 to 20 denier.
Various methods have heretofore been proposed to produce chitin fibers by wet spinning of a chitin solution. However, chitin fibers meeting both the requirements of tensile strength and thickness as described above have not yet been produced.
For example, U.S. Pat. No. 4,029,727 (corresponding to Japanese Patent Application (OPI) No. 133367/1976 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application")) describes chitin dope prepared by dissolving chitin in a trichloroacetic acid-containing solution, wet-spinning and cold-stretching, whereby high tensile strength chitin fibers are obtained. The thus obtained chitin fibers, however, are very thick. It is described in Example 2 that there could be obtained filaments having a tensile strength of 63 kg/mm.sup.2. This value corresponds to 5 g/d when calculated assuming that the density is 1.4. Although it is apparent that high tensile strength chitin fibers can be obtained, the diameter thereof is 0.25 mm as shown in Example 3. When calculated with the density as 1.4, the diameter corresponds to 618 denier. Thus, the fibers obtained in U.S. Pat. No. 4,029,727 do not meet the requirements as described hereinbefore.
Japanese Patent Application (OPI) No. 127500/1978 describes that chitin was dissolved in a solvent, such as dichloroacetic acid, to prepare a chitin dope solution. The prepared chitin dope solution was spun and stretched whereby fine chitin fibers were obtained. The tensile strength of the fiber, however, is low. That is, it is described in the examples that 3.0 to 3.5 denier of chitin fibers were obtained, but that the tensile strength was 1.2 to 1.5 g/d (a knot tensile strength of 0.6 to 0.7 g/d). Thus, it can be seen that the chitin fibers obtained in Japanese Patent Application (OPI) No. 127500/1978 do not meet the requirements as described hereinbefore.
U.S. Pat. No. 3,988,411 describes that chitin was dissolved in hexafluoroisopropyl alcohol or hexafluoroacetone sesquihydride, and the resulting solution was spinned to obtain chitin fibers. Sutures are disclosed as being one use of the chitin fiber. However, there is described no chitin fiber is described meeting the foregoing requirements.
The conventional chitin fibers, as described above, fail to meet both the requirements for tensile strength and thickness, that is, the thickness is too large when the tensile strength is sufficient, whereas when the thickness is sufficiently small, the tensile strength is poor. Thus, from such conventional chitin fibers, there cannot be obtained sutures which have suitable physical properties and ease of use, and which are useful for practical use.
Heretofore, chitin fibers having such sufficient tensile strength and flexibility that surgical sutures can be prepared therefrom, i.e., having a suitable thickness, have not yet been known.